Disk Galaxies as Cosmological Benchmarks: Cold Dark Matter versus Modified Newtonian Dynamics

Abstract

We discuss a comparison of models for the formation of disk galaxies both in a Universe dominated by cold dark matter (CDM) and one in which the force law is given by modified Newtonian dynamics (MOND). Our main aim is to address the claim made by McGaugh & de Blok that CDM suffers from severe fine-tuning problems, which are circumvented under MOND. As we show, CDM indeed requires some amount of tuning of the feedback efficiencies to obtain a Tully-Fisher relation (TFR) as steep as observed. However, that same model is in excellent agreement with a wide variety of additional observations. Therefore, the modest amount of feedback needed should not be regarded a fine-tuning problem. Instead, its requirement should be considered a generic prediction for CDM, which might be tested with future observations and with detailed modeling of feedback processes. We also show that galaxy formation in a MOND universe can not simultaneously reproduce the TFR and the lack of high surface brightness dwarf galaxies. We thus conclude that CDM is a more viable theory for the formation of disk galaxies than MOND.